During start-up of refiners of the type described above, the driving motor, which is generally a synchronous motor, creates rather strong oscillating moments. These oscillations pass through a resonance frequency of the mechanically coupled shafts immediately before the working number of revolutions, or the synchronous speed, has been achieved. The excitation moment from the synchronous motor increases substantially during this passage through the resonance frequency, and the maximum moment which develops can exceed about 200 kNm. In view of the magnitude of the maximum moment, the refiner must be overdimensioned in relation to what is required during the other refiner operation conditions.
The present invention solves this problem in that the oscillation moment can be reduced to a level close to that during normal operation conditions.